Yeah, but a real demon would never reveal his real name anyway. Although, presumably he'd have to know at least part of it to have summoned him in the first place. Isn't that the rules?

While I should note that this knowledge comes in spite of having never played the Pen and Paper game (in spite of owning several of the necessary tomes), in D&D you need not know the specific name of a demon in order to summon a demon. The name itself gives the one doing the summoning a greater degree of power in the ensuing negotiation and further grants the summoner the ability to summon a particular demon. Otherwise, you just summon a random demon from a given tier of the Demon social structure.

I'm sure other fantasy worlds have rules for Demon Summoning and all but D&D is the only one I know of off hand, mostly because I've never actually bothered reading the various rule books for other Fantasy RPGs.